1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing single crystals. More particularly, it relates to a method and an apparatus for manufacturing crystal by a weighing method wherein group III-V compound semiconductor single crystals of constant diameter are manufactured by the resistance-heated system.
2. Description of the Prior Art
In the conventional method for manufacturing the group III-V compound semiconductor single crystals, for example, GaP, GaAs, GaSb or InP, pulling from a melt is performed according to the Czochralski method, particularly, the Liquid Encapsulated Czochralski method (to be referred to as the LEC method for brevity hereinafter). During this procedure, various controls are performed according to fluctuations in the diameter of the obtained crystals so that the single crystals of constant diameter may be obtained. The control of fluctuations in the diameter of single crystals manufactured by pulling from a melt according to the LEC method is known to be performed by controlling the temperature of the melt. The conventional methods of heating control include the X-ray method wherein X-rays are projected on a single pulled crystal for observation of the diameter, or the weighing method wherein the weight of a single crystal during pulling is measured, so that fluctuations in the diameter of the single crystals may be corrected and single crystals of constant diameter may be manufactured. The weighing method, a particularly well-known prior art method, is disclosed in Japanese Patent Disclosure (Kokai) No. 50-131683, which corresponds to U.S. Pat. No. 4,258,003 and British Pat. No. 1,494,342 wherein the weight of a single crystal per unit length is measured during pulling from the melt in a crucible, a control circuit is operated according to fluctuations in the diameter, and power to be supplied to a work coil is controlled, so that single crystals of constant diameter may be manufactured. This weighing method is based on the fact that the measured weight of a single crystal is affected by the height of the meniscus which is formed between the crystal and the melt. According to this weighing method, the weight of this part is corrected to make the diameter of single crystals formed by pulling constant.
However, the prior art described above only shows experiments conducted for single crystals having diameters of 20 to 40 mm, and provides no teaching concerning single crystals having diameters of about 50 to 62 mm, which are most frequently used in industrial applications. It seems that some problems occur when prior art is directly applied to single crytals of larger diameters.
In the manufacture of the group III-V compound semiconductor single crystals, especially GaP single crystals, the diameter abruptly becomes small after it abruptly becomes very large. Once such a large fluctuation occurs, heating control for obtaining single crystals of constant diameter is difficult.
The phenomenon as described above is considered attributable to periodic disturbances of balance between the temperature of the crystal and temperature of the solid-liquid interface which is caused by the change in the heat conduction pattern at the solid-liquid interface when the crystal passes through the liquid encapsulant and is first brought into contact with the atmosphere during the pulling of single crystals from the melt.
When such an abrupt fluctuation in the diameter occurs, especially with a crystal having an index of plane of (100), the sectional shape changes from elliptical to square and facet growth becomes predominant.
When the temperature gradient in the axial and radial directions of the crucible holding the melt is increased in order to prevent such a great diameter fluctuation, the diameter fluctuation indeed decreases. However, the thermal stress increases, resulting in an increase in the etch pit density and cracking.
The LEC method, if used in order to manufacture the group III-V compound semiconductor signal crystals, especially GaP single crystals, cannot employ for the following reasons a known automatic diameter control device for achieving the PID control which is commonly used in crystallizing oxides or the like.
The first reason is attributable to the characteristics of the crystal growing process and resides in the fact that the change in the temperature of the melt by control of the heating device cannot catch up with an abrupt change in the diameter of the GaP crystals. An abrupt change in the diameter of the GaP crystals takes about 5 to 10 minutes. No problem occurs if the temperature of the melt changes at the same rate as this. However, in practice, the time constant of the thermal response of the melt is about 4 minutes. Even if the temperature control is performed, there is a delay of about 2 minutes until the melt temperature catches up with the fluctuation in the diameter, due to the control delay in the feedback system.
The second reason is that an abrupt increase or decrease of the diameter involves a sharp peak. Therefore, the differentiated value of the change of the diameter at this peak cannot be obtained, and the control operation thus becomes discontinuous.
The third reason is that the pulled single crystal is subject to the influence of the buoyancy of the liquid encapsulant. Accordingly, it is difficult to correctly measure the true weight of the crystal.
For the reasons described above, in some cases, the diameter of the single crystals being pulled is observed through a window formed in the furnace, and the temperature control of the heating device is manually performed according to a predetermined temperature program. However, with this manual control, correct control of the diameter may not be performed, internal distortion of the crystals increases, and single crystals of good quality may not be obtained. Therefore, an effective method for manufacturing single crystals, especially group III-V compound semiconductor single crystals, which utilizes the weighing method and which allows manufacture of single crystals of constant diameter by automatic control has not been available.